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358                          METALLURGY OF IRON AND STEEL.
as soon as it went beyond the watchful care of its parents and its nurses. Numerous cases can be cited of rails, plates, etc., containing from .10 to .35 per cent, of phosphorus, which have withstood a long lifetime of wear and adversity; but in the general use of such metal there has been such a large percentage of mysterious breakages that it seems quite well proven that the phosphorus and the mystery are the same.
Much information on the effect of phosphorus may be gathered from a study of high steels. A severe trial is put upon a cold-chisel or similar tool, and it is undeniable that each increment of phosphorus has its effect in rendering such a tool brittle. In this case the steel is quenched and it contains a considerable proportion of carbon, but there is no evidence to show that the effect of phosphorus is different when the carbon is high, even though it is more marked. Neither is there reason to suppose that quenching changes its nature, for with high-phosphorus steel of low carbon sudden cooling would rather counteract the influence of phosphorus than enhance it, since it tends to prevent the formation of coarse crystals.
It would seem, therefore, that the regularly increasing baneful-ness of phosphorus as the carbon is raised does not portray any change in nature, but that, although the effect of the metalloid in lower steels is obscured, its character is the same. No line can be drawn that can be called the limit of safety, since no practical test has ever been devised which completely represents the effect of incessant tremor. For common structural material the critical content has been placed at .10 per cent, by general consent, but this is altogether too high for railroad bridge work. All that can be said is that when all other things are equal safety increases as phosphorus decreases, and the engineer may calculate just how much he is willing to pay for greater protection from accident.
SEC. XYIIf.—Influence of copper.—The iron made from the ores of Cornwall, Pa., contains from .75 to 1 per cent, of copper^ and large quantities of rails have been made from this iron alone, but it has oftener been the custom at Eastern steel works to use from 25 to 50 per cent, of this iron in the mixture. Other 'deposits contain considerable quantities of this element, notably some beds in Virginia, while the ores of Cuba give an iron with about .10 per cent, of copper. Most of the Bessemer steels recorded in this .book contain from .30 to .50 per cent, of copper, while much of the open-